1. Field of the Invention
The present invention relates to a thermal recording system, and more particularly to a thermal recording system for electrically controlling the temperature of thermal elements.
2. Description of the Related Art
Thermal printers are widely used in devices such as computer systems, ticket vending machines and automatic fare systems. In particular, the thermal printer prints alphanumeric characters in dot matrix form on a medium coated with a thermally sensitive material. The thermal elements of the thermal printer comprise thin film deposited resistors through which an electric current is passed. The electric current is supplied in the form of pulses to heat the thermally sensitive material to a temperature at which it changes from a first state to a second state, i.e., a color. Thereby, the alphanumeric characters are printed on the paper by supplying the electric current to the proper elements.
Japanese Patent Disclosure No. 61-15469 (Kokai) titled "THERMAL PRINTING SYSTEM" published Jan. 23, 1986 to Nagato discloses an energy control system for a thermal printer. The thermal printer varies the pulse width of the electric current. The pulse width of the electric current is determined based on the pattern of the presence or absence of preceding dot images, succeeding (or following) dot images and any peripheral dot images.
The U.S. Pat. No. 4,695,528 titled "PROCESS FOR FORMING IMAGE USING BODY WITH REVERSIBLE FIXABLE AND TEMPERATURE-VARIABLE LIGHT EXTINCTIONS" issued Sep. 22, 1987 to Dabisch et al. discloses a thermally sensitive material. The thermally sensitive material exhibits maximum light extinction when cooled to a temperature T0 after heated to a temperature T2. Moreover, the thermally sensitive material exhibits minimum light extinction when cooled to a temperature T0 after heated to a temperature T1 (T0&lt;TI&lt;T2).
However, the thermal printer disclosed by Nagato cannot print correctly images on the thermally sensitive material, disclosed by Dabisch et al. That is to say, when attempting to heat the thermal element to a temperature T2, an actual temperature of the thermal element might not reach temperature T2 if the environmental temperature is too cool. In this case, the thermally sensitive material would not exhibit maximum light extinction. At other times when attempting to heat the thermal elements to temperature T1, the temperature of the thermal element may reach a higher temperature T2, because the thermal elements have accumulated heat. The environmental temperature may also be too hot. Therefore, the thermally sensitive material does not exhibit minimum light extinction.